Add support for bigWig

[danvk]

We already support bigBed. Adding support for bigWig shouldn't be hard.

[danvk]

cc @gibiansky

General outline for implementing this:

1. Create src/main/BigWig.js and src/test/BigWig-test.js, modeled after BigBed.js and BigBed-test.js
2. Create src/main/BigWigDataSource.js, modeled after src/main/BigBedDataSource.js
3. Create a visualization.

Steps 2 & 3 are mostly trivial. Hopefully step 1 isn't too bad since we already have code in place for parsing "bbi", i.e the commonalities between bigbed and bigwig.

[danvk]

@gibiansky just wanted to check in on this. Did you get a chance to play around with a bigwig parser?

[gibiansky]

@danvk No, not yet.